The purpose of this study is to define the metabolic abnormalities leading to hyperlipoproteinemia in the different types of familial hyperlipidemia. Three tyeps of hyperlipidemia will be studied: (a) familial hypercholesterolemia, (b) familial hypertriglyceridemia and (c) familial combined hyperlipidemia. Patients will be carefully selected through family screening to conform to these three classes of hyperlipidemia. Each patient will be admitted to the hospital for study of various parameters of lipid and lipoprotein metabolism. Comparable control patients without hyperlipidemia will be studied under identical conditions. The following studies will be carried out: turnover of low density lipoproteins, turnover of plasma triglycerides, synthesis of cholesterol and bile acids, adipose and liver lipase activities, and the rate of conversion of very low density lipoproteins to low density lipoproteins. The major aim will be to determine whether there is an overproduction of lipoproteins or a decreased removal to explain the increased plasma concentrations of lipoproteins. If there is an increased production, we will determine whether there is a simultaneous increase in synthesis of trilycerides or cholesterol. Thus, our goal is to define the abnormality in the different types of familial hyperlipidemia. This will be accomplished through the simultaneous study of lipid and lipoprotein parameters.